A calibrating method of this type is known from the OERLIKON B24 BLADE GRINDING MACHINE OPERATING INSTRUCTIONS, Date of Issue Mar. 9, 1999/B, Oerlikon Geartec AG, Zurich, which were delivered to VW Kassel together with Machine No. 289839. The contents of these operating instructions (hereinafter briefly referred to as O1) and in particular the parts quoted therefrom in the following are hereby incorporated in the present description by reference in their entirety.
The calibrating method initially referred to has been devised for a grinding machine having 5+1 NC axes of the type shown on page 9 of the O1 and, for greater ease of reference, in the appended FIG. 1. Such a grinding machine is used for sharpening cutting tools, such as a bar cutting blade 10 shown in the appended FIGS. 2 and 3, by means of a grinding wheel 12. The grinding machine has a table 17 on which a slide 18 is adapted to traverse back and forth along an X axis. A column 19 is adapted to reciprocate back and forth along a Z axis at right angles to the X axis. Provided on the column 19 is another slide 20 which is movable back and forth along a Y axis at right angles to the X axis and to the Z axis. The X axis, the Y axis and the Z axis form a rectangular coordinate system. Rotatably mounted on the slide 20 is the grinding wheel 12. Mounted on the slide 18 is a clamping fixture 21 for clamping the cutting blade 10. The clamping fixture 21 is mounted relative to the slide 18 by a positioning axis C—C and a positioning axis A—A normal to the positioning axis C—C. The X axis, the Y axis, the Z axis, the positioning axis A—A and the positioning axis C—C are not only able to position but also to move along CNC controlled curves.
According to appended FIG. 2 the bar cutting blade 10 has a shank 2 of rectangular cross-section and an end 3 essentially trapezoidal in longitudinal section. Provided at the end 3 are a rake surface C, on a left-hand flank 5 when viewing FIG. 2 a secondary clearance surface B extending from the rake surface C rearwardly, on a right-hand flank 6 when viewing FIG. 2 a primary clearance surface A extending from the rake surface C rearwardly, and on the upper end face a top surface K extending from the rake surface C rearwardly. Formed between the secondary clearance surface B, the top surface K, the primary clearance surface A and the rake surface C is a circumferential cutting edge 4. As shown in this Figure, shoulder surfaces A˜ and B˜, respectively, may be formed in the transition region from the primary clearance surface A and the secondary clearance surface B to the shank 2. Also as shown, a curved shoulder surface C˜ may be provided in the transition region between the rake surface C and the shank 2. The primary clearance surface A, the secondary clearance surface B and the rake surface C have each a facet AF, BF and CE, respectively. The facet angles amount to about 10 and are designated as YAF, YBF and YCF, respectively, in the appended FIG. 3 (with yBF being not visible in FIG. 3).
FIG. 4 shows a grinding wheel 12 suitable for grinding the bar cutting blade 10. The grinding wheel 12 has an axis of rotation S to which the grinding wheel is rotationally symmetrical. On one end face the grinding wheel 12 has a circular clamping surface 13 perpendicular to the axis of rotation S. Extending from the outer circumference of the clamping surface 13 is a conical grinding face Pp having a small diameter d1 and a large diameter d2, with the small diameter d1 being provided on the clamping surface 13. Adjoining the large diameter d2 of the conical grinding face Pp tangentially is a curved grinding face 14 of a radius Rs, which merges, again tangentially, with a cylindrical grinding face Ps. The cylindrical grinding face Ps gives way tangentially to a toroidal grinding face G which has a circular-arc-shaped cross-section with a radius of curvature Rg. The toroidal grinding face G extends radially inwardly, merging tangentially with a second conical surface 15 that is undercut relative to the toroidal grinding face G. The grinding wheel 12 is a diamond wheel, with the diamond grains being bonded by electrocoating. In FIG. 4 the position of the grinding wheel 12 (to be more precise: its finishing edge) in the direction of the Y and the Z axis is indicated by pY and pZ, respectively.
Appended FIGS. 5 and 6 show the clamping fixture in a front view and in a top plan view, respectively. The clamping fixture 21 is adapted to rotate about the positioning axis C—C and pivotal about the positioning axis A—A. Adapted to be held in the clamping fixture 21 is a left-hand bar cutting blade 10, as shown, or a right-hand bar cutting blade. The clamping fixture 21 has two stop surfaces 23, 24 for left- and right-hand bar cutting blades, respectively.
To sharpen bar cutting blades on the grinding machine, generation grinding and dual grinding processes are employed. The grinding wheel 12 described also enables form grinding (roughing) followed by generation grinding (finishing) of the surfaces of the bar cutting blade 10 without the need for changing the setup. Conveniently, the grinding wheel 12 rotates about the stationary axis of rotation 5, and the bar cutting blade to be sharpened is guided along the grinding wheel 12 while being adjusted to corresponding angles. The dual grinding process for bar cutting blades and a grinding wheel for carrying out the process are described in WO 02/058888 A 1.
From DE 29 46 648 02 a method of profiling and sharpening bar cutting blades is known which requires only a single pass for a complete grind.
The purpose of the calibrating method initially referred to is to detect deviations resulting from manufacturing and assembly inaccuracies upon a change of the clamping fixture 21 or the grinding wheel 12 and to give consideration, by means of calibration, to both the nominal data forming the basis for calculation and the instantaneous actual condition of the grinding machine when sharpening bar cutting blades. Calibration is also recommended after prolonged use of the grinding wheel, in order to compensate for wear-induced shifts (resulting from increased grinding forces).
Factors relevant for computation of the grinding path are:                relative distance of the two stop surfaces 23, 24 to the positioning axis C—C of the clamping fixture 21 (FIGS. 5 and 6):                    stop for left-hand cutting blades (aL)            stop for right-hand cutting blades (aR)                        position of grinding wheel 12 (finishing edge) in two axis directions (FIG. 4):                    Y axis (py)            Z axis (pZ)                        plus: dimensions of the (dual) grinding wheel (FIG. 4):                    radius of curvature of the finishing edge (Rg)            distance to roughing face (Ps)                        
The known calibrating method is described in detail in O1, pages 97–108, reference to which is herewith made to avoid repetitions.
This known calibrating method involves the step of producing a calibrating gage with fixed geometry on three surfaces for the grinding machine and supplying it along with the grinding machine. The three surfaces are the primary clearance surface A, the secondary clearance surface B, and the top surface K. A calibrating blade is ground in the machine in three steps or grinding stages and adjusted to the calibrating gage.
First Grinding Stage
The cutting blade is held clamped in the clamping fixture 21 by means of a gage block. Then the clamping height in the machine is measured (O1 page 100, section 6).
The top surface K is ground and measured in the machine with the cutting blade held clamped (page 103).
The measured value is input in the control unit. It effects a correction in the Y axis (O1, page 104, section 11).
Second Grinding Stage
The bar cutting blade 10 is ground in horizontal position (O1, page 104, section 14). The blade height is again measured in the machine (O1, page 105, section 17). The measured value is again input in the control unit (O1, page 105, section 19).
Third Grinding Stage
The machine grinds the primary clearance surface A or flank 6 and the secondary clearance surface B or flank 5 (O1, page 106, section 21).
The two clearance surfaces A and B are then measured outside the machine (O1, page 106, section 1) and compared with a calibrating gage (a so-called master calibrating blade). The measured values, that is, the deviations, are again input in the control unit. The machine is thus calibrated and set up.
This is a time-consuming method. Measuring in the machine is difficult and requires much practice. In the first two stages (first and second grinding stage) the grinding wheel oscillates over the top surface, which amounts to a grinding operation that does not occur in the production process, that is, the sharpening of bar cutting blades on the grinding machine. Furthermore, the known method necessitates three steps or grinding stages, including the first and the second grinding stage in which the top surface is ground twice to be able to determine the Y and Z component of an error, and a third grinding stage in which the two flanks are ground once to be able to determine the position of the clamping fixture relative to the C—C axis. During the first two steps the cutting blade cannot be removed from the machine for measuring, because in these steps the measurements are taken relative to the machine. With the known calibrating method, therefore, a clamping fixture error can be detected only in the third grinding stage. This means that the first and the second grinding stage may prove redundant in retrospect, because their results are of no use whatsoever because of an initially undetected clamping fixture error. Finally, for the dual method the known calibrating method either lacks sufficient precision or necessitates additional machine equipment.
It is an object of the present invention to provide a method of the type initially referred to in such a manner that it can be performed more easily and produces better results.